All websites are growing faster these days. Back when Facebook was started, by definition, you didn't have the social platforms that make sites and services go viral. Instagram is growing like gangbusters, but its growth is accelerated by platforms like Twitter, Facebook, Tumblr, the iPhone, etc. Again, the point isn't that it's not impressive, but that the landscape has changed so much that comparing today's growth rates with those of even a few years ago isn't an apples-to-apples comparison.

Google+ has an ad on every page of the world's most visited website. While it's not exactly trivial that Google+ seems to be showing such uptake (remember Google Buzz), you would expect that any well-designed product that Google is pushing so hard would be growing really really fast. What will make Google+ a success isn't the number of users but engagement—do people start spending their days on Google+ the way they do on Facebook now? It's too early to tell for sure, but the numbers so far look less encouraging.